suspended and unsprung mass of the car
Car enthusiasts, particularly those involved with racing, sometimes speak of “suspended” and “unsprung” weight (or mass). What do these expressions mean?
The spring is the suspension component that supports the vehicle and protects it, its occupants and the load from shocks. A car without springs would not be very comfortable and would soon collapse due to jolts and bumps. Horse-drawn carriages have used springs for centuries, and as early as the Model T Ford, metal springs were considered standard. Today, all cars and trucks travel on springs.
But when we say that a vehicle “travels on” springs, we are not really referring to the entire vehicle. The portion of any car or truck that is supported by the springs is its suspended weight, and the rest is its unsprung weight.
The difference between suspended and unsprung: suspended and unsprung weight
To understand the difference, imagine a car rolling forward until one of its front wheels hits a pothole large enough to move that wheel upward toward the car’s body. But as the wheel moves upward, the car body may not move much or not at all, because it is isolated from the wheel moving upward by one or more springs; The springs can be compressed to allow the car body to stay where it is while the wheel moves up and down under it.
That’s the difference: the car body and everything that is solidly attached to it has springs, which means that it is isolated from the wheels by compressible springs; the tires, wheels and everything directly attached to them are not suspended, which means that the springs do not prevent them from moving when the car goes up or down on the road.
Almost everything in a typical car has a suspended weight, because almost all parts are solidly attached to the body. As well as the body itself, which includes all other structural or frame components, the engine and transmission, the interior, and of course the passengers and cargo.
What about unsprung weight? The following are not suspended:
- Wheel bearings and hubs (the parts that wheels turn on)
- Brake assemblies (on most cars)
- On vehicles with a solid driveshaft, sometimes called a live axle, the axle assembly (including the differential) moves with the rear wheels and is therefore not suspended.
That’s not a long list, and particularly for cars with an independent rear suspension (that is, not a solid axle) unsprung weight represents only a fairly small portion of the total.
Semi-suspended parts: suspended and unsprung mass
There is a complication: part of the weight is lifted and part is released. Consider, for example, an axle that is attached at one end to the transmission and at the other end to the wheel (a “half axle”); When the wheel moves up and the body and transmission don’t, one end of the axle moves and the other doesn’t, so the center of the axle moves but not as much as the wheel does. Parts like this that have to move when the wheel does, but not that far, are said to be partially suspended, semi-suspended, or hybrid. Typical parts of semi-springs include:
- The springs themselves
- Shock absorbers and struts
- Control arms and some other suspension parts
- Half shafts and some transmission shafts
- Certain parts of the steering system, such as the steering knuckle
car suspended mass: car unsprung mass
Why does none of this matter? If a large part of a car’s mass is not suspended, it is more difficult to keep the tires firmly planted on the road while going over potholes because the springs have to apply more force to move them. Therefore, it is always desirable to have a high suspended to unsprung weight ratio, and is particularly important for vehicles that are intended to handle well at high speeds.
Thus, racing teams reduce unsprung weight, for example by using light but delicate magnesium wheels, and engineers try to design suspensions with the lowest unsprung weight possible. That’s why some cars like the 1961-75 Jaguar E have employed brakes mounted not on the wheel hub, but on the inner end of the axle shaft – it’s all an effort to reduce unsprung weight.
Note that unsprung weight or mass is sometimes confused with rotating mass because some parts (tires, wheels, most of the discs …